Force spectroscopy, adhesion forces measurement

Benoit, M., Gabriel, D., Gerisch, G.t., and Gaub, H.E. (2000). Discrete interactions in cell adhesion measured by single-molecule force spectroscopy. Nat. Cell Biol, 2, 313 317. 

Table 3 lists some ionization properties of functionalized gold-thiol monolayers and relevant alkylsiloxane monolayers together with the appropriate bulk values. Monolayers with carboxylate terminal groups show abnormal wetting behaviour, which makes it difficult to determine accurately their surface pKa values308. Apart from contact angle titration, other methods were also used to study proton transfer equilibria at the mono-layer surfaces, such as quartz crystal microbalance (Table 3, line 1), measurements of the adhesion force between the monolayer deposited at the surface of an AFM tip and the same monolayer deposited on the substrate (chemical force microscopy, Table 3, lines 3, 4, 15), FT-IR spectroscopy (Table 3, line 7), adsorption of polyelectrolytes (Table 3, line 5) and differential capacitance measurements (Table 3, lines 12, 13). 

Force spectroscopy, though originally conceived as a tool for calibrating the atomic force microscope, has become an invaluable tool for studying adhesive interactions on the nanometer scale [29 - 31]. In force spectroscopy the deflection of an atomic force microscopy (AFM) tip is measured as a sample is moved into and then out of contact with the tip. The characteristic hysteresis observed as the sample is retracted is due to adhesion between the tip and sample. The point at which the adhesion is broken and the AFM tip pulls off the sample surface is characterized by a sharp discontinuity in the... 

Force spectroscopy measurements between neutral polymer films (Fig 6(B)) show significantly higher adhesion than do measurements between oxidized polymer films (Fig. 6(C)). Statistical analysis of over 100 consecutive force curves for each type yielded an average adhesion force of 12.2 0.3 nN for the neutral polymer films and... 

Figure 9.2 Empirical force vs. distance (z) curve that reflects the type of interaction between the AFM tip and the sample during measurements using distinct imaging modes. (Reprinted with permission from Journal of Adehsion Science and Technology, Application of atomic force spectroscopy (AFS) to studies of adhesion phenomena a review by F. L. Leite, P. S. P. Herrmann et al., 19, 3-5, 365-405. Copyright (2005) Brill)...

Force measurements have been performed with scanning probe techniques to study protein properties and protein-surface interactions. Apart from specific recognition and specific interactions,"" single-molecule force spectroscopy,adhesion forces between ligand-receptor pairs, " and protein adsorption... 

When stimuli-responsive polymers are attached to a surface, changes in the physicochemical properties of the surface layer that result from an external stimulus can be assessed by AFM in situ. Micropattemed thermoresponsive polymers, such as poly(A-isopropylacrylamide) (PNIPAm), were characterized via AFM height imaging and the brush adhesiveness measured by force spectroscopy during... 

If we set out to unravel surface chemical functionalities with high spatial resolution down to atomic detail, we also encounter various practical (technical) problems. It is fair to say that the technique development for direct space analysis (again, we exclude Fourier space methods) is still lagging much behind. Chemical force microscopy can be considered as a first step in the direction of a true description of surface chemical functionalities with high spatial resolution in polymers, primarily based on the chemically sensitive analysis of AFM data via adhesion mapping. At this point the detailed theory for force spectroscopy is not developed beyond the description of London forces. The consideration of the effect of polar functional groups in force spectroscopy (similar to difficulties with solubihty parameter and surface tension approaches for polar forces, as well as specific interactions) is still in its infancy. Instead, one must still rely on continuiun contact mechanics to couple measured forces and surface free energies. 

Measurements were performed in dried nitrogen at 784 points over a mesh of 28 x 28 on the surface of the studied sample with averaging of the results of three measurements. The obtained data were used to plot maps of the adhesion forces. Because the frame size was 1000 nm and the number of pixels in a line during spectroscopy was 28, the spatial resolution during adhesion-force measurements was 1000 nm/28 = 36 nm. 

Subsequently, the forces of adhesion of the probe to the studied surface were measured by the force spectroscopy method (Fig. 9.3). 

On the basis of the force-spectroscopy data, we calculated the average force of adhesion of the probe to the studied surface (data were averaged over the analyzed area) and the adhesion force at each point and maps of the adhesion-force distribution were plotted before and after the sample modification (Figs. 9.2c. and 9.2d, respectively). It has been established that, after marking, the average adhesion force has decreased by 5% as compared to the initial value (5.3 0.03 and 5.6 0.04 nN, respectively). The RMS error of the adhesion-force measurements calculated for each of 784 points resulted in a value no higher than 0.04 nN at a confidence interval of 95%. 

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